Various multiple-antenna systems are known. For example, there are known systems that use transmit diversity, and there are many known schemes for multiple-input multiple-output (MIMO) transmission. It is typical in such systems to share the transmission power approximately equally among the various transmit antennas. For example, such equidistribution of power is advantageous in a two-antenna system because it makes it possible to use a balanced pair of power amplifiers, and to load the power amplifiers equally. FIG. 1, for example, schematically shows a forward-link transmission from a base station 10 to a mobile station 20. The transmission is made from an antenna array consisting of a main antenna 30 and a diversity antenna 40.
Certain problems may arise, however, when two (or more) base station antennas are used simultaneously for signal transmission. In such circumstances, there may be locations where the signals from the respective antennas undergo destructive interference. If mobile stations (or other terminals) happen to be at such locations, the result may be poor reception, leading to various consequences that are detrimental to the performance of the network.
For example, the 3rd Generation Partnership Project (3GPP) has defined a channel referred to as the fractional dedicated physical channel (F-DPCH), useful inter alia for transmitting uplink power control signals from the base station to the mobile stations in Wideband CDMA (WCDMA) networks. (In the following discussion, we will follow CDMA conventions to the extent of using the term “UE”—user equipment—to denote mobile stations and other user terminals.) One motivation for the F-DPCH is the conservation of downlink channelization codes. That is, users recover their own destined signals by despreading, using their own assigned orthogonal or quasi-orthogonal codes. But if a distinct code were assigned to each user for sending control messages on a forward-link dedicated channel, the remaining pool of available codes might be too small to serve the user population. The F-DPCH addresses this problem by reusing the same code among multiple users. Users who share the same code are configured by the network to have different frame timing.
The control information transmitted on the F-DPCH is a single symbol, i.e. the transmission power control (TPC) command, transmitted at regular intervals. The timing of these transmissions as defined, for example, in 3GPP 25.211, is illustrated in FIG. 2. Turning to FIG. 2, it will be seen that the illustrated example, one radio frame has a duration of 10 ms and comprises fifteen slots, numbered 0 to 14. Each slot has a duration of 2560 chips, each chip having a duration of 0.260 microseconds. Each slot is subdivided into a plurality of fields, each of which may potentially be used for transmitting a TPC symbol, i.e., a power-control command TPC_cmd having NTPC bits.
In current implementations, it is possible for multiple users to share the same F-DPCH, i.e., the same scrambling code, channelization code, and time alignment, provided that they use different slot formats. A “slot format” in this regard means a selection of a particular one of the fields within the slot in which to transmit the TPC symbol.
In the figure, the fields that are not used for transmitting the power-control command to a designated user, i.e. the fields shown as comprising a total length of NOFF1+NOFF2 bits, are labeled “Tx OFF”. By this is meant that there is no transmission in those fields for the designated user. If no other users are sharing the same F-DPCH, then the channel will be silent during the NOFF1+NOFF2 symbol intervals. However, as noted, those symbol intervals may be used for sending TPC symbols to other users that are sharing the same F-DPCH.
It is significant that because the conventional manner of transmitting on the F-DPCH involves sending individual symbols, it precludes the use, of space-time block-coding-based transmit antenna diversity (STTD), because such techniques must operate on pairs (Or greater-numbers) of distinct symbols.
In WCDMA as currently practiced, for example, when transmit diversity is enabled, the same F-DPCH symbol is transmitted simultaneously from both antennas of a two-antenna base station array. As noted, this may lead to destructive interference in some locations. As a consequence, some UEs may experience poor signal-to-noise ratio on the F-DPCH, which may possibly result in loss of synchronization. (Current WCDMA standards define the synchronization criterion with reference to the quality of the F-DPCH.)
There are, in fact, closed loop transmit diversity schemes which avoid the problem of destructive interference by dynamically adapting the phase of the transmissions from at least one of the antennas. However, these schemes also suffer from certain drawbacks that may make them disadvantageous under some circumstances. One drawback is that additional complexity is needed to support the feedback path for the antenna phases. A second drawback is that the phase control might not work well when the coherence time of the radio channel is short. Although 3GPP standards have adopted closed loop transmit diversity (CLTD) for some purposes, they have not adopted it for use on the F-DPCH.